Mitochondrial replacement therapy (creating so-called “three-parent babies”) could fail in some cases, because diseased versions of these vital cellular components might outcompete healthy replacements. That’s the finding of research led by scientists from Oregon Health and Science University and the Salk Institute for Biological Studies.

This therapy aims to prevent mitochondrial diseases from being transmitted to the next generation. Questions about the method’s safety and ethics are still being debated. It’s currently banned in the United States, but has been performed in Mexico, where there are no laws against such procedures.

The research, performed in embryonic stem cell cultures, indicates that donor mitochondria should be matched in growth potential with the recipient to prevent reversion to the diseased type, said the study, led by Shoukhrat Mitalipov of OHSU, along with colleague Paula Amato and the Salk Institute’s Juan Carlos Izpisúa Belmonte.

Mitochondria produce most of the body’s energy, and they contain their own DNA, replicating independently. Cells can contain hundreds or even thousands of mitochondria, which are inherited only from the mother.

Although their DNA constitutes a small fraction of the DNA contained in the nucleus, it’s essential for life. Diseased mitochondria can debilitate patients or even kill them.

Moreover, the inheritance is unpredictable; a woman with a mild form of the disease could give birth to a baby with a severe form. That’s because the populations of mitochondria in cells shift over time.

So if there’s a mixture of diseased and healthy mitochondria in cells, one or the other could come to predominate. Also, an egg cell could wind up by chance with a high proportion of diseased mitochondria.

Mitochondrial replacement therapy seeks to interrupt transmission of diseased mitochondria to the next generation. The procedure transplants nuclear DNA from an egg of a woman with mitochondrial disease into an enucleated egg cell from a woman with healthy mitochondria.

While a small number of diseased mitochondria ride along with the mother's nucleus, the preponderance of healthy mitochondria in the recipient egg is expected to be enough enough to prevent diseases.

The study transferred a “spindle,” an elliptical structure formed as the egg and sperm DNA merge into one. This method transfers less donor mitochondria than other methods,often less than 1 percent of the total in the donor egg cell, Mitalipov said in a Tuesday conference call.

However, the study found that in about 15 percent of the embryonic stem cell cultures, the ride-along mitochondria outcompeted the recipient mitochondria.

Mitalipov cautioned that this phenomenon has only been observed in cell cultures, so it’s unclear whether it actually occurs in people. Animal experiments failed to show this effect, he said.

A second mechanism was observed, the scientists said. Cells with certain mitochondrial haplotypes grew faster in cultures of embryonic stem cells. As the cells divided, the cultures eventually came to be dominated by cells with these mitochondria.

“There was no mutant mtDNA involved so far as we know, just two different healthy, wild-type mtDNA,” Mitalipov said. “And it seems like with the same nuclear genome, the cells having one haplotype grow much faster than the others, at least in embryonic stem cells. And if you have an equal number of cells, one mtDNA haplotype will take over, because they grow much faster.”

Four women with mitochondrial disease who have had children with the disease were recruited for the study, along with 11 healthy egg donors, Amato said in the conference call. Federal funding restrictions impeded getting a larger sample size.

Good to know

Jeanne Loring, who heads the stem cell program at The Scripps Research Institute, said the study provides “a useful piece of information.”

“This is the second paper showing that the composition of mitochondria can change radically after the transfer of a nucleus from one human oocyte to another,” Loring said by email “Dieter Egli at the New York Stem Cell Foundation reported this phenomenon earlier this year.

“This new paper from Mitalipov and colleagues suggests a logical reason for dominance of one mitochondrial type over another - it appears that the mitochondria that match the donor oocyte’s genome have an advantage,” Loring said.

“This means that it may make sense to match the genome type of donor and recipient oocytes when using this procedure to treat mitochondrial diseases. According to this study, this would increase the odds that the good mitochondria would thrive.”

Loring said the study left unaddressed the contributions of the male genome to the fertilized egg and embryo.

“If the male contribution to the genome is a very different haplotype from the female (who contributes all of the mitochondria), how do the mitochondria cope with mixed signals?” she asked.

Dusko Ilic, a stem cell scientist at King’s College London in the UK said the study provided a “quite positive” twist to developing mitochondrial replacement therapies.

“The finding is extremely valuable – we can, by choosing a healthy egg donor with a specific preferential amplification polymorphism in mtDNA, minimize a risk of reversal to the affected maternal phenotype, which could otherwise happen due to the presence of maternally inherited mitochondria incidentally transferred during the procedure,” Ilic said in comments provided by the UK’s Science Media Centre.

Salk Institute researchers discuss a 2015 study on how to prevent mitochondrial diseases from being inherited. The work continues with more research that identifies roadblocks to this therapy and how to overcome them.

Salk Institute researchers discuss a 2015 study on how to prevent mitochondrial diseases from being inherited. The work continues with more research that identifies roadblocks to this therapy and how to overcome them.

SCNT produced Dolly the cloned sheep in 1997, but this use, called reproductive cloning, is banned in the United States. Mitalipov and colleagues say they want to use the method only to make embryonic stem cells matched to patients, so replacement cells can be grown from them, which is called therapeutic cloning.

On Nov. 10, Mitalipov and and Salk Professor Joseph Ecker published a study that found genetic material normally discarded by human egg cells might be usable to create more embryos, which in theory could be brought to term.

That research was limited to eggs and days-old embryos because of ethical restrictions due to safety concerns. Likewise, the new research was limited to egg cells and embryonic stem cells derived from them. But if the methods can be shown to be safe, they could be incorporated into in vitro fertilization or mitochondrial replacement treatments.